Project Summary/Abstract Respiratory syncytial virus (RSV) is a major respiratory pathogen in human infants and elderly individuals. RSV infection is the principal cause of hospitalization of infants, and is a leading cause of infant mortality worldwide. In elderly adults, RSV infection accounts for 10-15,000 deaths annually in the US alone. Despite the high impact of RSV, there is presently no licensed vaccine despite decades of attempts. Inactivated RSV vaccines have resulted in vaccine-enhanced disease in infants, in which vaccinated infants have developed more severe disease than unvaccinated controls when subsequently infected with RSV naturally. While live attenuated vaccines have not caused enhanced disease, live vaccines have failed because of either poor immunogenicity or inadequate attenuation. We have developed a novel live vaccine (?Mnull RSV?) that appears capable of striking the elusive balance between immunogenicity and safety through its unique method of construction and delivery. Mnull RSV has a deletion of the gene for the M protein, which is responsible for reassembly of viral proteins once a cell is infected. Deletion of the M protein leaves Mnull RSV incapable of replicating beyond the first cycle, but still able to induce strong antibody responses and protect against an RSV challenge in a relevant (infant baboon) animal model. Because the entire gene for the M protein has been deleted, Mnull RSV cannot revert back to the wild type during infection, which is a problem with other live, attenuated mutant vaccines. Mnull RSV thus holds the advantages of live vaccines (broad humoral and cellular responses without adjuvants) and yet has a strong safety profile as it cannot re-assemble and spread, and cannot revert to a more aggressive phenotype. Having demonstrated successful protection by IP vaccination with an endotracheal tube, Heartland Vaccines LLC now seeks to demonstrate that we can achieve these results with a delivery mechanism appropriate for use in humans. Our Phase 1 hypothesis is that IP delivery of liquid Mnull RSV to infant baboons using existing commercially available nebulizers induces a high level of RSV neutralizing antibody and protective immunity without adverse clinical symptoms. In Specific Aim 1, we will determine the immunogenicity and optimal dose regimen of Mnull RSV in infant baboons, administered IP after aerosolization via a nebulizer. In Aim 2, we will challenge optimally vaccinated infant baboons with a wildtype RSV strain and assess the level of protection. If successful, Heartland Vaccines plans to exclusively license the Mnull technology (patent application in progress) and seek Phase II funding to establish protocols toward GMP manufacturing of Mnull RSV, and in addition, develop a dry powder formula and delivery device for Mnull to better enable storage, distribution, and worldwide vaccination.